1. Field of the Invention
The present invention relates to a method for measuring a piezoelectric constant of thin film shaped piezoelectric material, and more particularly to a method for measuring a piezoelectric constant of a thin film shaped piezoelectric material which can exactly measure a piezoelectric constant of a thin film shaped piezoelectric material by applying a uniform pneumatic pressure to the whole surface of the thin film shaped piezoelectric material regardless of a topology of the thin film, without causing a short in the thin film or a plastic deformation of the thin film.
2. Prior Art
Electronic devices such as micro-processors and memories have been miniaturized, so their performances have improved and their prices have strongly decreased. There is a similar need to miniaturize mechanical devices such as actuators, and an additional need for micro-actuators for future applications, for example, medical and biomedical. In general, micro-mechanical devices can be based on the electrostatic, piezoelectric, thermal, or electromagnetic principle.
A piezoelectric actuator converts mechanical energy into electrical energy via the piezoelectric effect, or converts electrical energy into mechanical energy via the inverse piezoelectric effect. Namely, the piezoelectric actuator converts electrical energy into mechanical energy by contraction or expansion of a piezoelectric material therein according to the orientation of applied voltage and internal polarization of the piezoelectric material. The construction or expansion of the piezoelectric material is not determined by the dimensions of the piezoelectric material, but by the magnitude and orientation of the applied voltage. Therefore, a piezoelectric thin film can be used to fabricate the micro-actuator. The maximum elongation of piezoelectric thin film is restricted by a breakdown field or a maximum stress. Hence, the micro actuator having the piezoelectric thin film is generally operated at a low voltage range below 10 V.
The piezoelectric actuator can be fabricated at a low cost by using the silicon technology. Various applications of piezoelectric thin film integrated on a silicon substrate are known. In most cases, ZnO is used as the piezoelectric thin film. However, lead zirconate titanate (PZT:Pb(Zr,Ti)O.sub.3) has a better piezoelectric property than ZnO. PZT is a complete solid solution of lead zirconate (PbZrO.sub.3) and lead titanate (PbTiO.sub.3). PZT having a cubic structure exists in a paraelectric phase at a high temperature. Orthorhombic structure PZT exists in an antiferroelectric phase, rhombohedral structure PZT exists in a ferroelectric phase, and tetragonal structure PZT exists in a ferromagnetic phase according to the composition ratio of Zr and Ti at a room temperature. FIG. 1 shows a binary phase diagram of PZT. Referring to FIG. 1, a morphotropic phase boundary (MPB) of the tetragonal phase and the rhombohedral phase exists as a composition which includes Zr:Ti at a ratio of 1:1. PZT has a maximum dielectric property and a maximum piezoelectric property at the MPB. The MPB exists in a wide region in which the tetragonal phase and the rhombohedral phase coexist, but does not exist at a certain composition. Researchers do not agree about the composition of the phase coexistent region of PZT. Various theories such as thermodynamic stability, compositional fluctuation, and internal stress have been suggested as the reason for the phase coexistent region.
Nowadays, PZT thin film is manufactured by various processes such as spin coating, organometallic chemical vapor deposition (OMCVD), and sputtering. It is reported that a method for measuring a piezoelectric constant of PZT thin film by measuring the amount of charge generated from the PZT thin film because of applying force to the PZT thin film (see: Measurement of Piezoelectric Coefficient of Ferroelectric Thin Films by K. Lefki and G. J. M. Dormans, J. Appl. Phys. 76 (3), Aug. 1, 1994, pp 1764-1767).
The electrical property of piezoelectric thin film is determined by an elastic constant, a piezoelectric constant, and a dielectric constant. Generally, the piezoelectric constant means an electrical activity level of piezoelectric material. For example, the piezoelectric constant means an expansion or contraction magnitude of a piezoelectric material corresponding to an applied electric field. An apparatus for testing properties of multilayer piezoelectric actuator such as piezoelectric constant, Young's modulus, and capacitance, is disclosed in U.S. Pat. No. 5,301,558 (issued to Jeffrey A. Livingstone et al.). FIG. 2 is a cross-sectional view for showing the apparatus for testing piezoelectric properties of the multilayer piezoelectric actuator, and FIG. 3 shows a block diagram of the control circuitry associated with the testing apparatus.
Referring to FIG. 2, the testing apparatus 10 has a piezoelectric actuator 15 having a housing 18 and a plurality of piezoelectric elements 20 formed in a column having a length, L.sub.1. Housing 18 includes a diaphragm 16 attached to an end of housing 18. Housing 18 protects actuator 15 and is used to mount actuator 15 to testing apparatus 10. The plurality of piezoelectric elements 20 are aligned with an axis 25. Each of piezoelectric elements 20 is shaped as a disk having a cross-sectional area, A. Metallic electrodes 30 are interleaved among the plurality of piezoelectric elements 20. Piezoelectric elements 20 are expanded axially, proportional to the magnitude of applied electrical energy. Thus, actuator 15 having piezoelectric elements 20 converts electrical energy into mechanical energy.
Referring to FIG. 2, testing apparatus 10 has a front plate 40, a rear plate 45, and an intermediate plate 50, which are made of hardened steel. Front plate 40 receives actuator 15 and rear plate 45 defines a cavity 55. A pneumatic cylinder 60 having a piston 65 therein is disposed within cavity 55 and is rigidly attached to intermediate plate 50. Front plate 40, intermediate plate 50, and pneumatic cylinder 60 define a central bore axially aligned with housing 18. A cylindrical plate 70 is disposed within the central bore of front plate 40, to lie adjacent to diaphragm 16 of housing 18. For example, cylindrical plate 70 is made of a high tensile steel and one end of cylindrical plate 70 defines a polished surface.
A steel pillow 75 is disposed within the central bore of intermediate plate 50 and pneumatic cylinder 60. One end of pillow 75 is adjacent to piston 65 of pneumatic cylinder 60. A load cell 80 is disposed between the other end of pillow 75 and cylindrical plate 70. Load cell 80 is formed in a shape of a cylindrical ring, and is coaxially disposed about axis 25. Pillow 75 and piston 65 define a small bore axially aligned with axes 25. A fiber optic sensor 85 is disposed within the small bore. Fiber optic sensor 85 has a sensor head located intermediate the polished surface of cylindrical plate 70. As shown in FIG. 2, a sensor housing 90 is fixedly attached to rear plate 45. Sensor housing 90 includes a micrometer 95 adapted to adjust the sensor head invariable proximity to cylindrical plate 70.
Referring to FIG. 3, a proportional pressure regulator 100 is connected to a source of pressurized air 105. Proportional pressure regulator 100 controllably supplies pressurized air to pneumatic cylinder 60. Proportional pressure regulator 100 applies an axial force to actuator 15 in response to the received pressurized air. A load member 110 measures the applied force on actuator 15 and responsively produces a force signal, F.sub.n. Load member 110 includes load cell 80 and a dual mode amplifier 115. Load cell 80 measures the applied force on actuator 15 and responsively produces a sense signal. Dual mode amplifier 115 receives and conditions the sense signal, to produce the force signal, F.sub.n, having a predetermined voltage range. An optical member 120 measures a linear displacement of actuator 15 and responsively produces a position signal, L.sub.n. Optical member 120 includes sensor 85 and associated signal conditioning circuitry 125. Sensor 85 emits random light onto the polished surface of cylindrical plate 70 and senses the reflected light, to responsively produce an optical signal. Signal conditioning circuitry 125 receives the optical signal and transposes the optical signal to the position signal, L.sub.n, having a voltage level proportional to the magnitude of the optical signal.
A high voltage power supply 130 delivers constant voltage levels to actuator 15. A sensing member 135 includes a current probe for measuring the electrical current flowing through actuator 15 and a voltage probe for measuring the voltage applied to actuator 15. A data acquisition board 140 is connected to signal conditioning circuitry 125, sensing member 135, high voltage power supply 130, and pressure regulator 100. A computer 145 receives the various signals via data acquisition board 140 and determines various performance properties of actuator 15. Also, computer 145 controls high voltage power supply 130 and pressure regulator 100 via data acquisition board 140. A plotter 150 and a printer 155 are connected to computer 145 for displaying test results.
It will be described below that a method for measuring the piezoelectric constant of the multilayer actuator by using the testing apparatus.
At first, the various test instruments such as load member 110, optic member 120, etc., which are associated with the piezoelectric constant, are initialized before the piezoelectric constant of the piezoelectric actuator 15 is measured. That is, no force or load is applied to actuator 15, amplifier 115 is adjusted to control the magnitude of the force signal, F.sub.n, to zero, and signal conditioning circuitry 125 associated with optic sensor 85 is initialized. Next, pneumatic cylinder 60 applies a predetermined force, for example 250 lbs., onto actuator 15. High voltage power supply 130 initially applies a voltage of 200V for a predetermined time of five seconds to actuator 15. Computer 145 controls high voltage power supply 130 to incrementally deliver voltages to actuator 15 at 100V increments up to 900V. The magnitude of the voltage delivered to actuator 15 is represented by a signal, V.sub.m. Actuator 15 is displaced a predetermined amount with each incremental increase in voltage. Optical sensor member 120 determines the axial displacement of actuator 15 and responsively delivers the position signal, L.sub.m, to computer 145, at each incremental change in voltage. Computer 145 acquires the test data and plots the data for later analysis. Once the data is acquired, the data is ready to be analyzed and evaluated. The piezoelectric constant is determined according to the relationship: ##EQU1## wherein m is an integer representing the number of measurements at each increment of applied voltage, N represents the number of active or polled piezoelectric discs, 0 represents the original value and f represents the final value.
However, in the above-described apparatus, after the predetermined force and voltage are applied to the piezoelectric actuator, the piezoelectric constant is calculated by measuring the displacement of the piezoelectric actuator according to the applied voltage. So, the piezoelectric constant measured by using the above described apparatus may vary according to the position and condition of the piezoelectric element and the arranged condition of the optic sensor member. Also, an error may occur in measuring the piezoelectric constant because the amount of displacement of the piezoelectric element is very small at a low voltage range. In addition, the construction of the apparatus may be complicated and the manufacturing cost may be high. Also, when a piezoelectric constant of single layer piezoelectric thin film is measured by using the apparatus, there may be a short in the single layer piezoelectric thin film, or a plastic deformation of the single layer piezoelectric thin film may occur. Furthermore, it is difficult to apply a constant force to the whole surface of the single layer piezoelectric thin film.